This invention relates to acoustic ink printing and, more particularly, to acoustic ink printing with hot melt inks. Acoustic ink printing is a promising direct marking technology because it does not require the nozzles of the small ejection orifices which have been a major cause of the reliability and pixel placement accuracy problems that conventional drop on demand and continuous stream ink jet printers have experienced.
As shown, FIG. 1 provides a view of a prior art acoustic ink printing element 10. As shown, the element 10 includes a glass layer 12 having an electrode layer 14 disposed thereon. A piezoelectric layer 16, preferably formed of zinc oxide, is positioned on the electrode layer 14 and an electrode 18 is disposed on the piezoelectric layer 16. Electrode layer 14 and electrode 18 are connected through a surface wiring pattern representatively shown at 20 and cables 22 to a radio frequency (RF) power source 24 which generates power that is transferred to the electrodes 14 and 18. On a side opposite the electrode layer 14, a lens 26, preferably a concentric Fresnel lens, is formed. Spaced from the lens 26 is a liquid level control plate 28, having an aperture 30 formed therein. Ink 32 is retained between the liquid level control plate 28, having an aperture 30 formed therein. Ink 32 is retained between the liquid level control plate 28 and the glass layer 12, and the aperture 30 is aligned with the lens 26 to facilitate emission of a droplet 34 of ink from the aperture 30.
The lens 26, the electrode layer 11 the piezoelectric layer 16, and the electrode 18 are formed on the glass layer 12 through known photolithographic techniques. The liquid level control plate 28 is subsequently positioned to be spaced from the glass layer 12. The ink 32 is fed into the space between the plate 28 and the glass layer 12 from an ink supply (not shown). The liquid level control/aperture structure 10 used in prior art was a piece of silicon 25 etched to form a thick wall enclosure in the outside and a much thinner aperture area in the inside as depicted in FIG. 1. Although this silicon liquid level control/aperture structure can be etched precisely, it is not practical to use it either in prototype, pilot or manufacturing scales due, to its high cost and fragility. When the requirement for the outside wall thickness is 356 μm, which is already thinner than the normal, silicon wafer of 500 μm the requirement for the inside aperture area is only 100 μm which is so vulnerable to breakage.
In addition to the cost issue and fragility problem, there is a thermal expansion mismatch between the silicon and the glass which is the substrate used to fabricate acoustic transducer, frensel lens and circuitry. The thermal expansion coefficient of silicon is 2.6 ppm/C while that of the glass (7059) is 4.6 ppm/C. The silicon liquid level control/aperture plate needs to be bonded 42 to the glass substrate 12 at elevated temperature which is required to cure the adhesive (Epon) during bonding. Warpage of the printhead structure is observed even for a 2 inch print head due to the thermal mismatch. The warpage will be tremendous when this structure is used for full width page printing. Therefore, what is needed is a structure for an AIP print head that solves the above-identified problems.